1. Field of the Invention
Embodiments of the present invention generally relate to coherent light sources such as lasers and, more particularly, to temporally and spatially decorrelating coherent light in an effort to provide intense and uniform illumination.
2. Description of the Related Art
The integrated circuit (IC) market is continually demanding greater memory capacity, faster switching speeds, and smaller feature sizes. One of the major steps the industry has taken to address these demands is to change from batch processing silicon wafers in large furnaces to single wafer processing in a small chamber.
During such single wafer processing the wafer is typically heated to high temperatures so that various chemical and physical reactions can take place in multiple IC devices defined in the wafer. Of particular interest, favorable electrical performance of the IC devices requires implanted regions to be annealed. Annealing recreates a more crystalline structure from regions of the wafer that were previously made amorphous, and activates dopants by incorporating their atoms into the crystalline lattice of the substrate, or wafer. Thermal processes, such as annealing, require providing a relatively large amount of thermal energy to the wafer in a short amount of time, and thereafter rapidly cooling the wafer to terminate the thermal process. Examples of thermal processes currently in use include Rapid Thermal Processing (RTP) and impulse (spike) annealing.
A drawback of RTP processes is that they heat the entire wafer even though the IC devices typically reside only in the top few microns of the silicon wafer. This limits how fast one can heat up and cool down the wafer. Moreover, once the entire wafer is at an elevated temperature, heat can only dissipate into the surrounding space or structures. As a result, today's state of the art RTP systems struggle to achieve a 400° C./s ramp-up rate and a 150° C./s ramp-down rate. While RTP and spike annealing processes are widely used, current technology is not ideal, and tends to ramp the wafer temperature during thermal processing too slowly and thus expose the wafer to elevated temperatures for too long a period of time. These thermal budget type problems become more severe with increasing wafer sizes, increasing switching speeds, and/or decreasing feature sizes.
To resolve some of the problems raised in conventional RTP type processes various scanning laser anneal techniques have been used to anneal the surface(s) of the substrate. In general, these techniques deliver a constant energy flux to a small region on the surface of the substrate while the substrate is translated, or scanned, relative to the energy delivered to the small region. Due to the stringent uniformity requirements and the complexity of minimizing the overlap of scanned regions across the substrate surface these types of processes are not effective for thermal processing contact level devices formed on the surface of the substrate.
Pulsed laser annealing techniques have been used to anneal finite regions on the surface of the substrate to provide well defined annealed and/or re-melted regions on the surface of the substrate. In general, during a pulsed laser anneal process various regions on the surface of the substrate are exposed to a desired amount of energy delivered from the laser to cause the preferential heating of desired regions of the substrate. Pulsed laser annealing techniques have an advantage over conventional processes that sweep the laser energy across the surface of the substrate, since the need to tightly control the overlap between adjacently scanned regions to assure uniform annealing across the desired regions of the substrate is not an issue, since the overlap of the exposed regions of the substrate is typically limited to the unused space between die, or “kerf” lines.
However, light waves produced by a laser often have high temporal and spatial coherence. Coherence is the property of waves that enables them to exhibit interference where at least two waves are combined to add constructively or subtract destructively depending on the relative phase between the waves. Temporal coherence characterizes how well a wave can interfere with itself at a different time and may be defined as the measure of the average correlation between the value of a wave at every pair of times separated by a given delay. Thus, a wave containing only a single frequency (a perfect sine wave or monochromatic light) is perfectly correlated at all times, while a wave whose phase drifts quickly will have a short coherence time. The most monochromatic sources are usually lasers, and higher quality lasers tend to have long correlation lengths (up to hundreds of meters). White light, which comprises a broad range of frequencies, is a wave which varies quickly in both amplitude and phase leading to a short coherence time (approximately 10 periods); thus, white light is usually considered as incoherent. Spatial coherence describes the ability for two points in the extent of a wave to interfere when averaged over time. More precisely, spatial coherence may be defined as the cross-correlation between two points in a wave for all times.
The coherence of laser beams manifests itself as speckle patterns and diffraction fringes, which suggest deviation from the desired uniform illumination in pulsed laser annealing and other applications. A speckle pattern is a random intensity pattern produced by the mutual interference of coherent waves that are subject to phase differences and/or intensity fluctuations. Because the surfaces of most materials are extremely rough on the scale of an optical wavelength (˜500 nm), coherent light from a laser, for example, reflected from such a surface results in many coherent wavelets, each arising from a different microscopic element of the surface. At any moderately distant point from the surface, the distances traveled by these various wavelets may differ by several wavelengths, and the interference of these wavelets of various phases results in the granular pattern of intensity called speckle. In other words, each point in the speckle pattern is a superposition of each point of the rough surface contributing with a random phase due to path length differences. Diffraction fringes are formed when light from a point source, such as a laser, passes by an opaque object of any shape.
Spatial coherence of light sources has been addressed by the use of random phase plates, also known as diffusers. Intended to scatter the light, optical diffusers increase the frequency of modulation due to interference, but they do not eliminate the interference. However, for pulsed laser annealing techniques and other applications, it is not sufficient to simply increase the frequency of modulation with a diffuser; the depth of modulation from coherence effects should be reduced, as well.
Accordingly, what are needed are techniques and apparatus for temporally and spatially decorrelating light from a coherent light source to provide incoherent light.